This invention relates to improvements in continuous-flow centrifuge systems. In particular, it relates to improvements in the mechanical design of conventional compensating rotors utilized in continuous-flow centrifuge systems.
The application of centrifugal force is widely used in the processing of blood and other biological suspensions. It provides a convenient means for sorting and classifying particulates on the basis of buoyant density differences and for retaining particles subjected to opposing hydrodynamic forces. An illustrative example of such usage is the continuous-flow washing technique for the deglycerolization of red blood cells.
Glycerol behaves as a cryoprotective agent, permitting the freezing and frozen storage of the red cell with minimal freeze-related damage. The concentrations of glycerol necessary to achieve this protective effect (viz., 20-40%), however, are not well tolerated by humans and the protectant must be removed from the thawed unit prior to infusion. This "washing" procedure may be accomplished either manually or by using one of several automated systems currently available.
Manual methods consist of an alternating sequence of saline dilution, centrifugal separation and supernatant expression. The technique is labor intensive and the quality of the product varies with the skills of the technician.
Commercial cell washing systems seek to reduce processing time per unit by half (to about 30 minutes) and improve quality consistency via automated centrifugation. The IBM system, for example, is an automated version of the discontinuous, batch-wise manual technique. Other illustrations are found in "flow-through" centrifuges, such as those marketed by Fenwal and Haemonetics, where centrifugal force is employed to retain the red cell mass in the periphery of a processing container spinning at 3000-4000 rpm while saline solutions of decreasing tonicity are passed continuously through cells at about 150-200 ml/min. in a direction countercurrent to the centrifugal field. In all these cases, the fluid exchange is effected in a more or less aseptic fashion by means of a rotary seal.
There are several disdvantages associated with the rotary seal arrangement in blood processing applications. The possibility of contaminants passing between the seal faces exists. Consisting, as it does, of an assembly of precisely machined components of specialty materials, the seal represents a major contribution to the fabrication and quality control costs of the blood processing container, which is designed to be a disposable item. In addition, the seal may impose flow limitations, and high shear rates at the seal juncture may damage the more labile blood components.
A recent advance in centrifugal apparatus development allows continuous flow blood processing without rotary seals. The "compensating rotor" is a mechanical device which permits the exchange of fluids between a stationary and rotating container via an integral tubing loop. The absence of the seal eliminates the contamination risk and permits substantially increased flow rates (&gt;1 liter/min.) with a corresponding reduction in processing time per unit of cells washed. Such an apparatus is useful not only in deglycerolization but also in various other modes of centrifugal blood processing, including component separation and pheresis applications.
The effect of the 2:1 relative rotation utilized in the operation of conventional twist compensating devices is well known in the art. One illustration of the application of this principle is the apparatus described in U.S. Pat. No. 3,586,413.
The N.I.H. blood centrifuge of the type described in the article by Y. Ito, et. al, "New Flow-Through Centrifuge Without Rotating Seals Applied to Plasmapheresis," Science 189, p. 999 (1975), employs 2:1 rotation to effect fluid transfer into a rotating processing container. Similarly, the same principle is utilized in the centrifugal liquid processing system disclosed in U.S. Pat. No. 3,986,442.
Although the conventional compensating rotor obviates certain problems associated with the earlier rotary seal systems, it possesses several disadvantages at typical operating speeds, which limit its effective use in the exchange of fluids between a stationary and rotating container. As a result of the 2:1 relative motion between the rotary components, and the associated mechanical stresses on the tubing loop, the effective lifetime of the fluid-carrying tubing loop is reduced considerably. Generally, this problem manifests itself in one of several possible ways.
One occurrence, of particular significance, relates to stress-related tubing loop failures, especially in regions of high flexural and torsional stress. These failures, which are associated with the untwisting process, are most severe in the center spindle and centrifuge cover areas of the compensating rotor.
In addition, when the tubing loop consists of discrete tubes which are used for fluid flow into and out of the rotating processing container, there is a tendency for the tubes to abrade one another in the regions of high centrifugal force, immediately above and below the rigid tube guide affixed to the rotating frame, where they are in close contact with one another. As indicated by Ito, the counterrotation of the tube guide facilitates the untwisting process of the rotor; however, the effective transfer of this contrarotary motion to the tubing loop depends on maintaining a good fit between the outer walls of the tubes and the inner surface of the guide through which the tubes pass. Furthermore, an improper fit between the inner surface of the tube guide and the exterior walls of the tubes may lead to slippage, causing excessive twisting or kinking of the tubes, particularly when thin walled tubing is used.
It is apparent that the major limitation of the conventional compensating rotor is its vulnerability at high rotational speeds, due to the 2:1 relative motion between the rotary components, and the associated mechanical stresses on the tubing loop. Since operating speeds of 3000-4000 rpm are required for effective and economical processing of blood, this is a significant limitation. The need for a continuous-flow centrifuge system capable of operating at 3000-4000 rpm is especially acute in the blood processing industry.
Accordingly, it is an object of the invention to provide an improved compensating rotor for use in a continuous-flow centrifugation system. More specifically, it is an object of the invention to overcome the aforementioned difficulties by providing means to relieve the mechanical stresses inherent in the operation of the conventional compensating rotor.
It is a further object of the invention to provide improved tube constraint fittings which constrain the lower central and upper central tubing loop segments in the areas of the center spindle and centrifuge cover, respectively, in such a manner as to minimize tubing wear and risk of rupture due to flexural fatigue.
It is still a further object of the invention to provide a novel tube guide insert with parallel channels which separate and constrain the peripheral segment of the tubing loop in such a manner as to minimize tubing abrasion and slippage due to centrifugal force.